To save power and ensure compatibility among devices built by different manufacturers, analog video signals are AC coupled. AC coupling implies that signal levels are not known by a video receiver, or change in time due to the ever changing content of the video signal. In applications where an analog video source is digitized, or several analog video signals are mixed, a need arises for the restoration and tracking of the DC level of the incoming video signal by the video receiver. In practice, the restoration and tracking of the DC level is accomplished by restoring the DC level of a certain content independent portion of the video signal to a known value via the use of a clamp circuit.
The ideal clamp circuit restores the desired value with a minimum error. Clamp noise (i.e., the line-to-line variation or offset in the level of the video signal) generates artifacts which tend to annoy the human eye when viewed. Equally important is the linearity of the circuitry (i.e., gain/programmable gain amplifier (PGA) etc.) which processes the incoming analog video signal prior to mixing or digitizing. Circuit topologies which lend themselves to the most linear signal processing with the least power consumption tend to make the design of a quiet clamp circuit difficult.
Conventional clamp circuits implement a high input impedance on a video receiver. Such a condition arises because a low input impedance combined with AC coupling capacitance creates a high pass filter. The high pass filter includes a high enough cutoff frequency to cause a drift of the voltage levels on the video receiver as the video content changes. The current drawn by the finite input impedance causes the DC level of the video signal to drift towards the voltage where the input impedance is terminated. Such a condition provides a line-to-line variation in the overall video signal as seen by the video receiver. To prevent drift in the video signal, the current from the clamp circuitry is increased to compensate for the current drawn by the finite impedance. The increased current temporarily restores the video signal to the appropriate level but cannot prevent the line-to-line fluctuation.
Maintaining a very high input impedance is difficult in designing a high bandwidth and linear circuit with low power consumption. In one example, a topology may include a video receiver with an operational amplifier implemented as a programmable gain amplifier and a clamping circuit. The operational amplifier of a PGA is configured as a non-inverting operational amplifier. The input of the operational amplifier presents a high impedance. Linearity is difficult to achieve, especially using low power supplies and large input levels since an input node between the operational amplifier and the clamping circuit tries to track the incoming signal. A designer may have to use complicated rail-to-rail input operational amplifier and/or consume a lot of current to get the performance needed for high linearity and bandwidth applications such as high definition television (HDTV). Another approach may involve implementing the operational amplifier in an inverting configuration. In principle, such a topology may yield very linear and low power designs at lower supply voltages since both inputs of the operational amplifier are held at some reference voltage (i.e., VOP) and move very little. However, with an inverting configuration, the effective input impedance of the video receiver is the resistance connected to the input of the operational amplifier. The impact of the input resistor is minimized by selecting a very large value. The selection of the input resistor with a large resistance value implies that to get any gain out of the operational amplifier in an inverting configuration, a feedback resistor has to be very large. Implementing a feedback resistor with a high resistance value combined with any parasitic capacitance at the input of the operational amplifier reduces the bandwidth and makes it difficult to design a linear gain stage.
It would be desirable to implement a method and/or apparatus to allow the simultaneous design of a quiet clamp and very linear analog front end for a video receiver.